1. Field of the Invention
The present invention relates to a method of manufacturing a diaphragm for an electroacoustic transducer which operates in accordance with the electrodynamic principle and has portions with different thicknesses made from a thermoplastically deformable material having a constant thickness.
2. Description of the Related Art
The behavior of the diaphragm is of particular importance for the manner of operation of each electroacoustic transducer because the behavior determines to a very significant extent the transmission properties of the transducer. In addition to the material properties, the behavior of the diaphragm depends very significantly on the shape of the diaphragm. In electroacoustic transducers which operate in accordance with the electrodynamic principle, a diaphragm shape has been found generally very useful as it is schematically illustrated in FIG. 1 of the drawing.
The center of the diaphragm is dome-shaped and is surrounded by a toroidally-shaped part to which is connected, in turn, a rim for fastening the diaphragm to the magnetic system. The moving coil which moves in a radially extending magnetic field is mounted underneath the dome.
The object of all structural measures is to achieve a piston-like movement of the moving coil at all frequencies and amplitudes. For this purpose, it is necessary to construct the individual diaphragm portions differently in accordance with their function.
Thus, the central dome should be of a construction which is as stiff as possible in order to suppress even at high frequencies the creation of vibration modes which would otherwise lead to drops in the frequency pattern.
The toroidally-shaped portion determines the resiliency and is frequently provided with indentations, or so-called crimpings, which extend tangentially relative to the inner diameter. For this reason, this portion is generally referred to by the term "crimped zone". The softer this crimped zone, the better the sensitivity in the low frequency transmission range will be.
These different requirements with respect to the various portions of a diaphragm of an electroacoustic transducer can be met in an optimum manner if each portion is constructed differently.
However, when manufacturing a diaphragm, these different requirements are frequently not taken into consideration. Thus, the diaphragm material, which usually is present in the form of a foil, is pressed in a stamping mold while pressure and heat are applied. After a cooling phase, the foil is removed and subsequently the final diaphragm is punched out. Except for their different shapes, this method of manufacturing type does not make a difference between the dome-shaped zone and the crimped zone.
Therefore, methods were frequently proposed which provide a stiffening of the dome-shaped zone. For example, by applying a second layer, a stiffening of the dome-shaped zone can be achieved. Thus, EP 0 446 515 A2 even proposes to provide the concave side of the dome with a metal fabric.
Another possibility of achieving a stiff dome area is to construct the diaphragm with different thicknesses. For example, in the construction of loudspeakers, diaphragms are used which have partial areas with different thicknesses. DE 38 38 853 C1 discloses a special thickness distribution of the central portion and the conical portion in order to ensure favorable transmission properties.
It is also possible to divide the diaphragm and to manufacture the central portion of a thicker material, as disclosed in EP 0 204 386 A1.
Another possibility of increasing the stiffness of the central dome-shaped portion is to provide this portion with a special shape, for example, by a central indentation, as shown in FIG. 1 of EP 0 137 624 A2. DE 43 29 637 A1 describes the formation of ribs. The central dome-shaped portion is initially manufactured larger than desired and is subsequently provided in a second deformation process with randomly produced stiffening ribs.
The methods described above are time-consuming and, thus, expensive.